This project involves the description of the basic mechanisms whereby energy metabolism is controlled, and the pertubations in these mechanisms that occur in old-age. Work has focussed on the role of the calcium ion in regulating pyruvate dehydrogenase activity in brain mitochondria and this has relevance to the synthesis of neurotransmitter substances. The study has involved both experiments with isolated mitochondria, in which the pCa of the medium is varied, and experiments with synaptosomes, in which cystosolic free calcium ion concentration is elevated indirectly, by depolarization of the plasma membrane. This mimics the excitation of nerve and leads to increases in the active form or pyruvate dehydrogenase. Cytosolic free calcium concentrations have been measured directly in these studies by the use of the fluorescent chelating agent Quin-2. Responses of both the pyruvate dehydrogenase system and of Quin-2 fluorescence to the inhibitors ouabain, KCN, oligomycin and ruthenium red have been investigated, in order to assign quantitative significance to the role of calcium ions and to adenine nucleotides in mediating between membrane depolarization and dehydrogenase activation. Other studies have focussed on calcium transport by brain mitochondria by both electrogenic uptake and electroneutral release pathways.